Need help replacing leaky shower

I feel I have watched every possible youtube video, but I cannot find my type of value for my shower.

I have disassembled it this so far, but cannot go any further. I have tried rotating the white part (in both directions), but nothing happens. I have tried loosening the metal part (that looks like a nut), but it wont budge. I have tied pulling the metal part straight off, but nothing.

I have taken pictures in the hope that someone can help identify it, and tell me how to fix/replace it.